I have two odd problems with synced content on my android. Both have been happening for months across multiple server releases and android app releases.
First, when I have synced content on a device (phone or tablet), if I open the app and browse the media on my server and play an item (which is also synced on my device), the app correctly starts playing the local, synced content instead of streaming. However, if I stop playing it in the middle, it does not update the position on the server. If I try to play it again, it starts from the beginning. However, if instead of browsing the server media, if instead I choose "This Device" and browse to it directly, it correctly updates the position where it stopped.
Second, on both my phone and tablet (phone is running Android 6.0.1 and tablet is running 5.1.1; both internal storage, no SD card; Both have latest android app and server is running latest), when an item is finished (like a tv episode), it says "Preparing synced content" like the server should transcode the next one. However, nothing happens and it stays that way every time I hit sync. Most of the time under details it still shows the item that is finished and not the new one. The way that I have gotten to fix this is to make a "major" sync change on the device, such as deleting a show or adding a show. It does not matter if it is the same show or not. As soon as I delete one show or add another one, it seems like is clears whatever problem is was and updates all of them. This seems like a silly hack, however.